Method for Controlling a Variable Event Valvetrain

ABSTRACT

A method to start an internal combustion engine having a variable event valvetrain is provided. The method includes increasing the speed of said internal combustion engine during a start from a stopped position after a request to start said internal combustion engine. The method may further include increasing an intake valve lift amount, of at least a cylinder during said start while said internal combustion engine speed is increasing. The method may further include only enabling fuel flow to said cylinder when said intake valve lift amount reaches a predetermined level.

FIELD

The present description relates to a method for improving enginestarting and stopping for an internal combustion engine having avariable event valvetrain. The method may be particularly useful forhybrid vehicles where there may be frequent engine starting andstopping.

BACKGROUND

One method to control intake and exhaust valve operation during engineoperation is described in U.S. Patent Publication No. U.S. 2003/0106515.This method presents a means to operate a variable event valvetrainduring vehicle starting and stopping. The method attempts to controlvalve operation after a request to stop or start the engine has beenmade.

During a stop sequence, the method reduces valve lift and fuel in anattempt to gradually reduce engine speed without disturbing the driver.By controlling the inducted air amount and the fuel amount, the methodattempts to control engine torque so that the engine will decelerate toa stop in a more controlled manner.

The method also attempts to control engine starting by adjusting valvelift. In one embodiment the valve lift is moved to a desired amount justafter an engine is stopped and then is held constant during a subsequentengine start. In a second embodiment valve lift is adjusted to apredetermined position just before a start and then is held constantduring the start. These valve lift strategies attempt to provide asmooth start when an engine is automatically started.

In addition, the method also attempts to reduce engine emissions afteran engine is stopped by setting the valve lift to a predeterminedposition.

The above-mentioned method can also have several disadvantages.Specifically, the method controls valve timing and fuel amount duringengine stopping without regard to combustion stability. By notrecognizing that combustion stability may be affected by reducingcylinder air amount, the method may produce misfires and increase engineemissions.

In addition, vehicle emissions may be higher than desired during anengine start since the above-mentioned methods may allow oxygen to bepumped through the engine as the valves follow a constant lift command.In other words, the engine is started in a manner that is similar to anengine having a fixed valve lift amount and similar emissions may beexpected. For example, during starting, engine position may not be knownuntil the engine has rotated to a certain position. As a result,cylinder fueling may be delayed so that the cylinders may be fueled in apredetermined combustion order (e.g., 1-3-4-2 for a four cylinderengine). By delaying fueling and thus delaying combustion, air may bepumped through the engine to a catalyst, during at least a portion ofthe starting sequence. The air may cool the catalyst and may also supplyoxygen to catalyst sites that may have otherwise reduced NOx.Consequently, lower catalyst temperatures and fewer reduction sites maydecrease catalyst efficiency during an engine start.

The inventors herein have recognized the above-mentioned disadvantagesand have developed a method of controlling a variable event valvetrainthat offers substantial improvements.

SUMMARY

One embodiment of the present description includes a method to control avariable event valvetrain during stopping an internal combustion engine,the method comprising: reducing a valve lift amount of at least acylinder in response to a request to stop said engine; and stopping fuelflow to said cylinder when said reduced valve lift amount reduces thecylinder air charge of said cylinder below a predetermined amount.

By reducing valve lift after a request to stop an engine and bydeactivating fuel when the air amount inducted into a cylinder is belowa level that likely supports a desired combustion stability level,engine emissions and undesirable operator perceptions may be reduced.For example, valve lift and cylinder fueling can be adjusted in acontrolled manner to reduce engine torque during an engine stopsequence, at least during some conditions. However, stopping fuel flowwhen a cylinder inducted air amount reaches a predetermined level (e.g.,an air amount that can result in a desired level of likely combustionstability) can reduce engine emissions since engine misfires may bereduced, thereby decreasing the amount of exhausted hydrocarbons. Inaddition, audible engine noise and engine torque may be more uniformsince combustion may be more consistent.

Further, another embodiment of the present description includes a methodto start a variable event valvetrain internal combustion engine, themethod comprising: increasing the speed of said internal combustionengine during a start from a stopped position after a request to startsaid internal combustion engine; and increasing an intake valve liftamount of at least a cylinder during said start.

By increasing valve lift as engine speed increases during an enginestart, engine emissions and the amount of oxygen pumped to an exhaustsystem catalyst during engine starting may be reduced. Variable eventvalvetrains may be commanded to a low lift position, including zerolift, for at least a portion of the interval between engine stop and apredetermined engine speed (e.g., idle speed). By operating the variableevent valvetrain at a low lift position the amount of air pumped throughthe engine may be reduced. As engine speed increases, and as engineposition is determined, valve lift may be increased so that combustionmay be initiated in selected cylinders. In this way, lower valve liftamounts can reduce oxygen flow to a catalyst during a portion of astarting sequence and higher valve lift amounts can be used to increasecylinder charge so that torque can be generated during another portionof the starting sequence.

In addition, during a start, fuel flow can be stopped until an inductedair amount reaches a level that reduces the chance of misfires. This mayfurther reduce engine starting emissions.

The present description may provide several advantages. Specifically,the approach may improve engine emissions by reducing the amount ofoxygen that may be pumped to a catalyst between the time that the engineis commanded to stop and the time the engine is restarted. In addition,the method may be used to reduce driver perceivable disturbances,namely, engine torque and audible engine noise. Further, the method mayalso provide shortened engine start and/or engine stop times which maylower emission levels.

The above advantages and other advantages, and features of the presentdescription will be readily apparent from the following DetailedDescription when taken alone or in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages described herein will be more fully understood by readingan example of an embodiment, referred to herein as the DetailedDescription, when taken alone or with reference to the drawings,wherein:

FIG. 1 is a schematic diagram of an engine;

FIG. 2 a is an illustrative valve event profile for an example variableevent valvetrain;

FIG. 2 b is another illustrative valve event profile for an examplevariable event valvetrain;

FIG. 3 a is an example plot of cylinder air flow versus valve lift at aconstant engine operating conditions;

FIG. 3 b is another example plot of cylinder air flow versus valve liftat constant engine operating conditions;

FIG. 4 is an example plot of signals that may be of interest during asimulated engine stop and start sequence;

FIG. 5 is an example plot of signals that may be of interest during analternate simulated engine stop and start sequence;

FIG. 6 is a flow chart of an example stopping sequence for a variableevent valvetrain engine;

FIG. 7 is a flow chart of an example starting sequence for a variableevent valvetrain engine; and

FIG. 8 is a flow chart of an example cylinder deactivation sequence fora variable event valvetrain engine.

DETAILED DESCRIPTION

Referring to FIG. 1, internal combustion engine 10, comprising aplurality of cylinders, one cylinder of which is shown in FIG. 1, iscontrolled by electronic engine controller 12. Engine 10 includescombustion chamber 30 and cylinder walls 32 with piston 36 positionedtherein and connected to crankshaft 40. Combustion chamber 30 is showncommunicating with intake manifold 44 and exhaust manifold 48 viarespective intake valve 52 an exhaust valve 54. The exhaust valve isoperated via cam 53 and the intake valve is operated via variable eventactuator 51. Alternatively, both exhaust valve 54 and intake valve 52may be operated by variable event actuators. The variable event valveactuator may be a mechanical apparatus that is controlled by electricalor hydraulic components, or alternatively, the valve actuator may beelectrically or hydraulically driven, or may be comprised of acombination of mechanical, electrical, and/or hydraulic components,electromechanical valves for example. In addition, the valve actuatormay be capable of adjusting valve lift, valve phase or the combinationof phase and lift. Some actuator designs may allow zero valve lift, aminimum lift, negative valve overlap between intake and exhaust valves,positive valve overlap between intake and exhaust valves, and/orcombinations of lift and phase adjustment amounts. U.S. Pat. No.6,145,483 describes one example of a variable valve actuator and ishereby fully incorporated by reference.

Intake manifold 44 is shown having fuel injector 66 coupled thereto fordelivering liquid fuel in proportion to the pulse width of signal FPWfrom controller 12. Fuel is delivered to fuel injector 66 by fuel system(not shown) including a fuel tank, fuel pump, and fuel rail (not shown).Alternatively, the engine may be configured such that the fuel isinjected directly into the engine cylinder, which is known to thoseskilled in the art as direct injection. In addition, intake manifold 44is shown communicating with optional electronic throttle 125. Further,an air mass sensor (not shown) may be located upstream of throttle 125,if desired.

Distributorless ignition system 88 provides ignition spark to combustionchamber 30 via spark plug 92 in response to controller 12. UniversalExhaust Gas Oxygen (UEGO) sensor 76 is shown coupled to exhaust manifold48 upstream of catalytic converter 70. Alternatively, a two-stateexhaust gas oxygen sensor may be substituted for UEGO sensor 76.Two-state exhaust gas oxygen sensor 98 is shown coupled to exhaust pipe49 downstream of catalytic converter 70. Alternatively, sensor 98 canalso be a UEGO sensor. Catalytic converter temperature is measured bytemperature sensor 77, and/or estimated based on operating conditionssuch as engine speed, load, air temperature, engine temperature, and/orairflow, or combinations thereof.

Converter 70 can include multiple catalyst bricks, in one example. Inanother example, multiple emission control devices, each with multiplebricks, can be used. Converter 70 can be a three-way type catalyst inone example. Alternatively, the converter may be a NOx trap, Hydrocarbontrap, oxidation catalyst, or a selective oxidation catalyst.

Controller 12 is shown in FIG. 1 as a conventional microcomputerincluding: microprocessor unit 102, input/output ports 104, andread-only-memory 106, random-access-memory 108, keep-alive-memory 110,and a conventional data bus. Controller 12 is shown receiving varioussignals from sensors coupled to engine 10, in addition to those signalspreviously discussed, including: engine coolant temperature (ECT) fromtemperature sensor 112 coupled to water jacket 114; a position sensor119 coupled to an accelerator pedal; a measurement of engine manifoldpressure (MAP) from pressure sensor 122 coupled to intake manifold 44; ameasurement (ACT) of engine air charge temperature or manifoldtemperature from temperature sensor 117; and an engine position sensorfrom a Hall effect sensor 118 sensing crankshaft 40 position. In apreferred aspect of the present description, engine position sensor 118produces a predetermined number of equally spaced pulses everyrevolution of the crankshaft from which engine speed (RPM) can bedetermined.

Referring to FIG. 2 a, illustrative valve event profiles 201 and 200 forrespective intake and exhaust valves of a variable event valvetrain areshown. The x axis represents crankshaft angle over a portion of afour-stroke cylinder cycle. Crankshaft angle markings are reference totop-dead-center (TDC, 0°) on the illustrated cylinder. The y axisindicates the valve lift of intake and exhaust valves. The intake andexhaust valve lift profiles illustrate the amount of intake or exhaustvalve lift at a particular crankshaft position. The figure shows thatvarious intake valve lift amounts may be achieved at different valveactuator operating positions. A high lift intake profile is illustratedby curve 201. Further, this design may include a zero lift position,whereby valve opening may be inhibited. By adjusting the valve lift, theinducted cylinder air amount may be varied at a given engine operatingcondition. Consequently, engine torque may be regulated by adjustingvalve lift and/or phase.

In an alternate embodiment, the exhaust valve may also have a means forvariable actuation. In this configuration, the exhaust profile may besimilar to the illustrated intake profile. Alternatively, an adjustableexhaust valve profile may be constructed such that it is different thanthe intake valve adjustment profile.

Referring to FIG. 2 b, illustrative valve event profiles 204 and 203 forrespective intake and exhaust valves of an alternate variable eventvalvetrain that has a minimum valve lift and valve phase control isshown. This plot is similar to that of FIG. 2 a, but a minimum liftprofile 205 is shown in a second phased position 206. In one example,where a valve actuator device has to operate with at least a minimumlift amount, low amounts of cylinder air charge may be achieved byphasing the minimum valve lift profile so that the peak valve lift isnear TDC 206. In other words, cylinder air amount may be adjusted bymoving intake valve opening (IVO) and/or closing positions (IVC). Also,the amount of intake and exhaust valve overlap may also be regulated byadjusting intake and/or exhaust valve phase to further reduce the amountof air that may be pumped through a cylinder. Valve phase adjustmentmechanisms, absent lift control, may allow a simpler actuator design toreduce inducted air amount to a level approaching the zero liftvalvetrain.

To increase or decrease the amount of inducted cylinder air, valve liftand/or phase may be adjusted individually or simultaneously. Also, thevalve operating strategy can be based on the respective valve durationsand/or range of phase authority, for example.

Referring to FIG. 3 a, an example plot of cylinder air flow versus valvelift at constant engine operating conditions is shown. The plotillustrates the relationship between valve lift and cylinder air flowfor a valve actuator that is capable of varying valve lift from zerolift to a higher lift amount. The x axis of the plot represents a valvelift amount while the y axis represents flow rate into the cylinder. Thefigure shows line 301 that illustrates a linear relationship betweenvalve lift and flow into a cylinder. However, the lift/flow relationshipmay change due to intake valve/port geometry and the pressure ratiobetween the intake manifold and the cylinder. This plot shows thatcylinder air flow can be reduced to zero and illustrates that the amountof oxygen pumped to a catalyst during engine start or stop may bereduced by adjusting valve lift.

Referring to FIG. 3 b, another example plot of cylinder air flow versusvalve lift at constant engine operating conditions is shown. Similar toFIG. 3 a, the x axis represents valve lift amount while the y axisrepresents cylinder air flow. An alternate actuator design may producethis air flow versus lift profile by being limited to a minimum valvelift and by allowing intake valve opening and closing to be phased(moved) with respect to a crankshaft position. Varying valve lift at afixed valve phase can produce a sloped line similar to line 305. If thevalve actuator phase adjustment can limit flow through the cylinder, thecylinder flow characteristics can transition from line 305 to line 303so that cylinder flow is reduced when a minimum valve lift level isreached. On the other hand, it may be possible that adjusting the valvephase reduces the flow through the cylinder to some near constant amountas shown by line 302. By changing the valve phase relative to acrankshaft position, cylinder air flow may be reduced from line 305 toline 302 since the valve opening and closing positions can determine theamount of inducted air. This plot shows that valve lift may be used inconjunction with valve phase as an alternate method to reduce cylinderair flow during engine stopping and starting.

Referring to FIG. 4, an example plot of signals that may be of interestduring a simulated engine stop and/or start is shown. An engine startmay include a cranking period (Ref. FIG. 5), an assisted nearly constantrate of engine speed increase from a stop (Ref. FIG. 4( b)), or acylinder initiated direct start. A starting interval may be defined in anumber of ways including: a period between the point where enginerotation begins and when engine speed reaches a predetermined speed(e.g., idle speed); a period between the point where engine rotationbegins and when engine speed reaches a predetermined speed under powerof the engine; a period between the point where engine rotation beginsand when engine speed has passed through a predetermined speed apredetermined number of times; or a period between the point whereengine rotation begins and when engine speed has reached a predeterminedspeed for a predetermined period of time.

Graph (a) represents an example engine stop request signal. This signalmay be generated by an operator acting on a switch or automatically by acontroller that monitors vehicle operating conditions and determineswhen to stop and/or start the engine, a hybrid powertrain controller forexample. The high portion of the signal represents a command or requestto stop the engine while the low signal portion represents a request tostart the engine or to continue to operate the engine. The timing of theengine stop request relative to the other signals of FIG. 4 isillustrated by vertical lines T₁ and T₄.

Graph (b) illustrates an example engine speed trajectory during arequest to stop and start an engine. In some hybrid vehicleconfigurations, engine speed may be controlled using the secondary motoror independently from the secondary power plant (e.g., an electric orhydraulic motor). U.S. Pat. Nos. 6,176,808 and 6,364,807 describe ahybrid powertrain that may be capable of controlling engine speed via asecondary motor and independent engine and motor speed control. Thepatents are hereby fully incorporated by reference. This engine speedtrajectory represents one of several trajectories that may be possibleby controlling engine speed in a hybrid powertrain. In one example, anelectric motor and a transmission can be used to control engine speedduring stopping and starting. In addition, the valve lift and phase maybe controlled with respect to the engine position and speed so that theinducted air amount may be regulated. In the figure, engine decelerationand acceleration are controlled during respective start and stopsequences. The engine speed and valve timing may be adjustedsimultaneously to provide a desired cylinder air amount.

Graph (c) shows three example cylinder air flow amounts over a number ofcombustion events during engine starting and stopping. During enginestopping a fixed cam mechanical valvetrain can induct air similar to theway that is described by line 402. Since the valve timing is fixed, thecylinder air flow may be largely a function of engine speed. Thecylinder air flow described by this line is the highest of the threeexamples. Cylinder air flow using a fixed cam mechanical valvetrainduring a start may be described by line segment 409. FIG. 4 shows theengine stop request at a low level T₄, indicating start and operate theengine, and engine speed increasing after the engine stop request hasbeen withdrawn. The cylinder and engine air flow increases as the enginespeed increases. If the cylinder air flow increases while combustion isinhibited, oxygen pumped through the engine may cool and/or occupycatalyst sites that may be used to reduce NOx. Consequently, theefficiency of the catalyst may be reduced. On the other hand, ifcombustion is initiated at low cylinder air flows misfires may result.Therefore, it may be desirable during a start to limit cylinder air flowand inhibit combustion until a desired level of combustion stability maybe attained.

Line 403 describes an example of cylinder air flow control using avariable event valve control mechanism that may be limited by certainlift amplitude and/or by a phase control constraints. For example, avalve actuator may be limited to 1 mm of valve lift during an intakestroke. On the other hand, another valve actuator may be limited to acertain valve phase amount at a constant lift amount. The valve actuatorlift/phase amount control signal described by the fourth graph (d) showsan example trajectory for reducing cylinder air flow during an enginestop. After a request to stop the engine, the valve lift and/or phasemay be adjusted to reduce cylinder air flow as shown in the graph (d).The effect of engine speed and valve lift/phase on cylinder air flow canbe seen in line 403 which shows two distinct segments that can describeair flow during an engine stop. The first segment after a request tostop the engine describes the effect of engine speed reduction and valveadjustments. The second distinct line segment occurs after the valvelift/phase described by the graph (d) is complete (i.e., at some reducedlift and/or altered phase amount). This line segment shows that thevalve actuator lift and/or phase limitations may not completely stopengine air flow through the engine while the engine is rotating, butthat cylinder air flow can be reduced compared to a fixed timingmechanical valvetrain.

During engine starting, a lift/phase limited valve actuator may becontrolled such that the actuator can be indexed from a partial orminimum flow position to another partial or full range flow position. Bykeeping the actuator at a minimum flow position the air flow through theengine may be reduced during a start. For example, line 410 shows onepossible air flow reduction strategy during starting. Cylinder air flowmay be reduced while the engine speed is below a target or desiredamount, and then increased to a partial or full amount of the actuatorrange as the engine speed approaches a target speed, idle speed forexample. This strategy can lead to cylinder air flow that may berepresented by the two segment line 410.

Cylinder air flow for a valve actuator that may be capable of reducingcylinder air flow to near zero during an engine stop may be described byline 401. This line shows an engine air flow amount that can be afunction of engine speed and valve lift and/or phase. When the actuatorreaches the minimum position illustrated in the graph (d), engine airflow is reduced to or near zero. Line 401 illustrates that it maypossible to reduce the cylinder air amount to a level that is lower thanthe amount described by line 402 (fixed cam valvetrain) and line 403(limited range valve actuator).

Engine starting may be further improved by allowing little or no airflow through an engine during starting. As described above, air flowthrough an engine during starting can reduce catalyst efficiency. Line408 illustrates the result of one engine air flow amount controlstrategy that may be used to reduce the amount of oxygen that may bepumped to a catalyst during starting. Specifically, the air flow may belimited until a desired or target engine speed. Then, air flow may beincreased until a desired engine or cylinder air flow amount isachieved.

Graph (d) illustrates one example of a valve lift trajectory that may beused to regulate engine and/or cylinder air flow. In this example, thevalve lift command is reduced from an initial value at T₁ to a value atT₃. Alternately, the lift reduction may begin at a time before or afterthe engine request stop time. That is, the engine stop may be delayeduntil a predetermined valve lift adjustment has been achieved, ifdesired. In addition, the valve lift and/or phase amount does not haveto be linearly ramped to a reduced position. Rather, the lift reductionand/or phase adjustment may be a step or stepped transition, anexponential decay transition, or a transition that may be a combinationof the previously mentioned methods.

As mentioned above, increasing valve lift during an engine start may bedelayed to reduce engine air flow. The engine start illustrated by FIG.4 delays the valve lift adjustment for the period between T₄ and T₅ andcompletes the adjustment by T₇ where the desired engine speed isreached. In this example the delay time before valve lift adjustment(T₅-T₄) can be determined from the amount of time it can take toaccelerate the engine from a stop to the desired start speed (T₄ to T₇),minus the time that it can take to move the valve lift actuator. Similarto the stop sequence, the valve lift amount does not have to be linearlyramped to a higher lift amount during a start. The lift may be a step orstepped transition, an exponential rise transition, or a transition thatmay be a combination of the before mentioned methods.

As described above, depending on the valve actuator design, it may alsobe possible to adjust valve timing to control engine and cylinder airflow. Valve phase may be adjusted in a manner that is similar to thatillustrated by graph (d). However, valve timing may be advanced orretarded to reduce the amount of engine air flow depending on the basevalve timing and the phaser range of authority.

Graph (e) shows an example of fuel delivery control during enginestopping and starting. Fuel flow is stopped at T₂, a location that maybe coincident with an engine or cylinder air amount that designates alower boundary of air necessary for a desired level of combustionstability. That is, fuel flow may be stopped when combustion stabilityis likely to be less than a desired level, thereby reducing misfires. Inthis example, fuel can be stopped at a cylinder air amount identified atlocation 405 for a valvetrain that may be capable of reducing cylinderair flow to near zero, at location 406 for a valvetrain capable oflimited cylinder air flow reduction, and at location 407 for avalvetrain having fixed cam mechanically actuated valves. Fuel controlduring a start is also shown in graph (e). Fuel may be enabled at T₆where an increase in valve lift can allow a cylinder to induct an airamount that may produce a desired level of combustion stability.Delaying fuel until a level of combustion stability may be attainablemay reduce engine emissions and driver disturbances since the number ofcylinder misfires may be reduced. In this example, fuel may be delayedduring a start for a cylinder air amount identified at location 412 fora valvetrain that may be capable of reducing cylinder air flow to nearzero, at location 411 for a valvetrain capable of limited cylinder airflow reduction, and at location 413 for a valvetrain having a fixed cammechanically actuated valves.

An alternative method to start a variable event valvetrain can be toincrease engine speed from a stop to a predetermined speed (e.g., idlespeed) while the valve lift or phase is set to a reduced amount andwhile fuel flow is stopped. At or near the predetermined desired enginespeed, fuel flow may be activated and valve lift may be increased orvalve phase may be adjusted so that combustion may be initiated in oneor more cylinders. In other words, at an engine stop, valve lift mayinitially be set to zero or to a partial lift amount, when the enginereaches a predetermined speed the valve lift may be set to an increasedpartial lift amount or to a full lift amount. Intake and/or exhaustvalves may be controlled in this manner, but engine starting may be moredifficult if flow through exhaust valves is reduced since more exhaustresiduals may be included in the cylinder mixture. In this way, valvetiming can reduce or stop oxygen flow to an exhaust catalyst so thatcatalyst efficiency may be increased.

Note: A hybrid powertrain may have two or more potential torque outputdevices and is hereby defined as the combination of an internalcombustion (IC) engine with a secondary power system. For example, ahybrid powertrain may comprise a combination of an IC engine and anelectric motor, an IC engine and a hydraulic power system, an IC engineand a pneumatic power system, an IC engine and one or more energystorage flywheels, and various combinations of the before mentionedsystems. In addition, during an engine stop it is not necessary that thevalve lift/phase be adjusted from a maximum to a minimum valve lift. Inother words, the valve lift can be reduced during the stop sequence froma first lift amount to a second lift amount. Also, the effect that thevalve lift/phase amount adjustment has on engine air flow may depend onengine speed, valve geometry, and initial and/or final lift amount.Likewise, during an engine start it is not necessary to increase thevalve lift amount from a minimal amount to a maximal amount. The valvelift may be increased from a first amount to a second amount.Furthermore, the valve lift and/or phase of exhaust valves may also beadjusted during an engine stop sequence, but it may be preferential toreduce exhaust valve lift after engine speed is at or near zero sincedecreasing exhaust valve lift may increase combustion residuals andreduce combustion stability.

Referring to FIG. 5, an example sequence that illustrates an alternativesimulation of an engine stop and start is shown. The signals and graphsare similar to those shown in FIGS. 4. However, FIG. 5 illustrates adifferent engine starting method. In particular, engine starting withthe assistance of a conventional starter motor is shown.

Graph (a) shows an example engine stop request signal. As mentionedabove, the request to stop may be generated in a number of waysincluding by a driver or by a hybrid powertrain controller.

Graph (b) shows engine speed during a stop and a start. The engine stopsequence is the same as in FIG. 4, but in this example no engine speedcontrol is provide by a secondary motor (e.g., an electric or hydraulicmotor).

Engine starting speed is shown on the right hand side of the graph (b).The figure shows engine speed increasing and leveling off to a crankingspeed (i.e., the cranking period) by way of a starter motor. Crankingoccurs approximately during the period between T₄ and T₆. After fuel isintroduced at location T₆ the engine speed begins to increase from theresulting in-cylinder combustion. After run-up (i.e., the intervalbetween engine cranking speed and engine idle speed where the engine isaccelerating) the engine speed stabilizes at a predetermined level, idlespeed for example. However, it is not necessary that the engine speedremain at idle speed, the engine speed may change after the run-upperiod in response to operator demand.

Graph (c) shows cylinder air flow over a number of combustion eventsduring engine starting and stopping. Cylinder air flow lines 501, 502,and 503 show cylinder air flows for a valvetrain that can reduce flow toor near zero, a fixed cam mechanically driven valvetrain, and valvetrainactuator having a limited range of authority, respectively. Fuel flow isstopped at a cylinder air amount that is represented by the respectivecylinder air flow curves at locations 505, 507, and 506.

Similar to the sequence illustrated by FIG. 4, engine air amount can bereduced during a stop sequence so that combusted gases continue to heatand provide exhaust gases to a catalyst. The combusted gases flow to thecatalyst until a desired predetermined level of combustion stability maynot be attained. Further, air flow may be reduced until a low lift or adesired valve phase is reached.

When starting by a starter cranking method, the cylinder air amount forrespective valvetrains may be illustrated by lines 508, 509, and 510.Cylinder air flow for a valvetrain having a fixed cam mechanicallyactuated valves corresponds to line 509, a valvetrain actuator havinglimited range of authority may be represented by line 510, and avalvetrain actuator capable of cylinder air flow to or near zero may berepresented by line 508. Fuel flow is started at a cylinder air amountthat is represented by the respective cylinder air flow curves atlocations 513, 512, and 511.

Graph (d) illustrates an example valve actuator lift and/or phase amountduring engine stopping and starting. Cylinder air flow reduction byadjusting a valve actuator begins at T1, coincident with the engine stoprequest, and ends at T₃.

On the right hand side of the graph (d), valve actuator adjustment isshown during a start. In this example, the valve adjustment is delayedfor a time after the request to stop the engine has been withdrawn. Thedelay period duration may be zero or it may be a function of the time torecognize engine position, engine position at start, time to pressurizethe fuel delivery system, engine temperature, or any other engine orvehicle operating condition, for example.

Graph (e) of FIG. 5 illustrates the timing of enabling fuel flow duringengine stopping and starting. During this example engine stoppingsequence, fuel is stopped at location T₂ which corresponds to a cylinderair charge at location 505 of the curve that represents one method ofcontrolling a valve actuator that may be capable of zero or near zerocylinder air flow. Locations 506 and 507 represent air charge amountsthat are equivalent to location 505 using different valve actuationmethods, but the time that it takes to achieve these levels of cylinderair charge may be increased since cylinder air amount is being reducedat a lower rate. Consequently, in other examples, fuel flow deactivationmay be delayed by the amount of time that it may take to reach thecylinder air amount that represents a desired level of combustionstability. This method can be used to decrease engine torque whileproviding a combusted mixture to the catalyst, and may reduce the amountof air that may be pumped to the catalyst during an engine stop.

Fuel flow enablement during a start is shown by the right hand side ofgraph (e). At location T₆ fuel is activated, this location correspondsto the cylinder air amount 512 that can provide a desired level ofcombustion stability. Cylinder air amounts at locations 513 and 511 arethe same level of cylinder air amount at location 512, but the cylinderair charge levels are achieved before the time that the cylinder aircharge is achieved at location 512. In other words, during cranking andrun-up more air may flow through an engine having a fixed cammechanically actuated valvetrain or through a limited range adjustablevalvetrain than through a valve actuator that may be capable of zero ornear zero cylinder air flow. Reducing the air flow through the engineduring cranking and run-up may reduce engine emissions. For example,fuel may be delayed during a start so that the engine controller hastime to determine engine position and deliver a fuel amount to aselected cylinder. However, by delaying fuel flow during a start, somecylinders may pump air though the engine thus cooling and/or oxygenatingthe catalyst, thereby potentially reducing catalyst efficiency during asubsequent restart.

Referring to FIG. 6, a flow chart of an example engine stopping sequencefor a variable event valvetrain engine is shown. During an engineshutdown (i.e., an engine stop sequence) some engines are stopped byimmediately stopping fuel flow and spark to the engine cylinders. Afterfuel flow is stopped the engine can continue to rotate as the enginespeed decreases. As a consequence, air that has not participated incombustion may be pumped from the intake manifold to the exhaust systemand through a catalyst. This may increase engine emissions when theengine is restarted since the air may cool the catalyst and/or theoxygen in air may occupy catalyst sites that otherwise could be used toreduce NOx.

In step 601, the routine determines if a request to stop the engine hasbeen made. If a request has not been made to stop the engine the routineexits. The routine of FIG. 6 can be repeatedly executed at predeterminedtimes or in response to an engine or controller operating event so thatvalve adjustments may be readily made. If a request has been made theroutine continues to step 602.

In step 602, engine speed can be reduced, and cylinder air flow may alsobe reduced by adjusting a valve actuator mechanism. In one embodiment,valve lift and fuel amount may be adjusted to reduce the cylinder chargemass, thereby, reducing the available cylinder torque. In anotherembodiment, valve phase (i.e., the valve opening and/or closingpositions relative to a crankshaft position) and fuel may be adjusted toreduce the cylinder charge mass. In yet another embodiment, valve lift,valve phase, and fuel amount may be adjusted to reduce the cylindercharge mass. The adjustments to valve lift and phase may be madesimultaneously or consecutively. The fuel adjustment may be madeproportionally to the cylinder air amount adjustment or it may be afunction of engine operating conditions, such as engine temperature andtime since start, for example.

A number of different methods may be used to adjust the valve actuator(e.g., valve lift and/or valve opening and/or closing phase) so thatcylinder air charge and/or engine torque may be lowered during an enginestop. In one embodiment, the valve lift may be reduced from a high liftlocation by a predetermined rate, 0.05 mm/sec or 0.05 mm/combustionevent for example. In another embodiment, the valve opening and closingpositions may be retarded or advanced by 100 crankshaft angle degreesper second, for example, so that the inducted air charge may be lowered.In yet another embodiment, the valve lift or phase may be adjusted infurther response to engine operating conditions, barometric pressureand/or desired torque for example.

In one example, intake valve timing and lift may be adjusted whileexhaust valve timing can be fixed so that exhaust valve opening andclosing positions are known. In this example and other examples, themethod described in U.S. patent application Ser. No. 10/805642 can beused to determine cylinder air amount after a request to stop an engineand the application is hereby fully incorporated by reference.Individual cylinder air amounts can be determined from cylinder pressurewhich can be related to engine torques by the following equation:

${I\; M\; E\; {P_{cyl}({bar})}} = {\left( \frac{\Gamma_{brake} - \begin{pmatrix}{\Gamma_{friction\_ total} +} \\{\Gamma_{pumping\_ total} + \Gamma_{accessories\_ total}}\end{pmatrix}}{{Num\_ cyl}_{Act}} \right)*\frac{4\; \pi}{V_{D}}*{\frac{\left( {1*10^{- 5}\mspace{14mu} {bar}} \right)}{N/m^{2}} \cdot {SPKTR}}}$

Where IMEP_(cyl) is the cylinder indicated mean effective pressure,Γ_(brake) is engine brake torque, Γ_(friction—total) is the total enginefriction torque, Γ_(pumping—total) is the total engine pumping torque,Γ_(accessories—total) is the total engine accessories torque,Num_cyl_(Act) is the number of active cylinders, V_(D) is thedisplacement volume of active cylinders, SPKTR is a torque ratio basedon spark angle retarded from minimum best torque (MBT), i.e., theminimum amount of spark angle advance that produces the best torqueamount. By reducing the engine brake torque, engine speed may be reducedduring a stop.The term SPKTR can be based on the equation:

${SPKTR} = \frac{\Gamma_{\Delta \; {SPK}}}{\Gamma_{M\; B\; T}}$

Where Γ_(ΔSPK) is the torque at a spark angle retarded from minimumspark for best torque (MBT), IMBT is the torque at MBT. The value ofSPKTR can range from 0 to 1 depending on the spark retard from MBT.

Individual cylinder fuel mass can be determined, in one example, foreach cylinder by the following equation:

m _(f) =C ₀ +C ₁ *N+C ₂ *AFR ² +C ₄ *IMEP+C ₅ *IMEP ₂ +C ₆ *IMEP*N

Where m_(f) is mass of fuel, C₀-C₆ are stored, predetermined, regressedpolynomial coefficients, N is engine speed, AFR is the air-fuel ratio,and IMEP is indicated mean effective pressure. Additional or fewerpolynomial terms may be used in the regression based on the desiredcurve fit and strategy complexity. For example, polynomial terms forengine temperature, air charge temperature, and altitude might also beincluded.

A desired air charge can be determined from the desired fuel charge. Inone example, a predetermined air-fuel mixture (based on engine speed,temperature, and engine load), with or without exhaust gas sensorfeedback, can be used to determine a desired air-fuel ratio. Thedetermined fuel mass from above can be multiplied by the predetermineddesired air-fuel ratio to determine a desired cylinder air amount. Thedesired mass of air can be determined from the equation:

m_(a)=m_(f)·AFR

Where m_(a) is the desired mass of air entering a cylinder, m_(f) is thedesired mass of fuel entering a cylinder, and AFR is the desiredair-fuel ratio.

Some variable event valve trains may vary the valve closing positionwith the valve lift height, reference FIG. 2 a for example. In othervariable event valve trains the valve opening and valve closinglocations may vary with valve lift height. In yet other variable eventvalve trains, the valve opening position may vary with valve lift.Consequently, a method that can determine valve timing using a varietyof valve actuators can be desirable.

In one example, valve timing and lift that can be used to induct thedesired amount of air into a cylinder may be determined by the methoddescribed in U.S. Pat. No. 6,850,831 which is hereby fully incorporatedby reference. Intake valve closing position can influence cylinder airamount, at least during some conditions, because inducted cylinder airamount can be related to the cylinder volume at IVC and the pressure inthe intake manifold. Therefore, the cylinder volume that can hold thedesired mass of air in the cylinder may be determined so that the IVClocation may be established. In other words, the cylinder volume duringthe intake and/or compression stroke that can hold the desired air mass,at a given intake manifold pressure, may be resolved into a uniquecrankshaft angle, the angle describing IVC. The cylinder volume at IVCfor a desired mass of air entering a cylinder may be described by thefollowing equation:

$V_{a,{I\; V\; C}} = \frac{m_{a}}{\rho_{a,{I\; V\; C}}}$

Where ρ_(a,IVC) is the density of air at IVC, V_(a, IVC) is the volumeof air in the cylinder at IVC. The density of air at IVC can bedetermined by adjusting the density of air to account for the change intemperature and pressure at IVC by the following equation:

$\rho_{a,{I\; V\; C}} = {\rho_{amb} \cdot \frac{T_{amb}}{T_{I\; V\; C}} \cdot \frac{P_{I\; V\; C}}{P_{amb}}}$

Where ρ_(amb) is the density of air at ambient conditions, T_(amb) isambient temperature, T_(IVC) is the temperature of air at IVC, P_(IVC)is the pressure in the cylinder at IVC, and P_(amb) is ambient pressure.In one example, where IVC occurs before bottom-dead-center (BDC),pressure in the cylinder at IVC can be determined by differentiating theideal gas law forming the following equation:

${\overset{.}{P}}_{I\; V\; C} = \frac{{{\overset{.}{m}}_{cyl} \cdot R \cdot T} - {P_{I\; V\; C} \cdot \overset{.}{V}}}{V}$

Where P_(IVC) is cylinder pressure, V is cylinder volume at a particularcrankshaft angle, R is the universal gas constant, and m is flow rateinto the cylinder estimated by:

${\overset{.}{m}}_{cyl} = {\frac{{C_{D} \cdot {A_{value}(\Theta)}}{\cdot P_{run}}}{\sqrt{R \cdot T}} \cdot \left( \frac{P_{cyl}}{P_{run}} \right)^{\frac{1}{\gamma}} \cdot \sqrt{\frac{2 \cdot \gamma}{\gamma - 1} \cdot \left( \frac{P_{I\; V\; C}}{P_{run}} \right)^{\frac{\gamma - 1}{\gamma}}}}$

Where C_(D) is the valve coefficient of discharge, A_(valve)(θ) iseffective valve area as a function of crankshaft angle θ, P_(run) is themanifold runner pressure which can be assumed as manifold pressure atlower engine speeds, and Υ is the ratio of specific heats. CD iscalibratible and can be empirically determined.

The effective valve area, A_(valve)(θ), can vary depending on the valvelift amount. The valve lift profile can be combined with the valvedimensions to estimate the effective area, A_(valve)(θ), via thefollowing equation:

A_(valve)(Θ)=L(Θ)·2·π·d

Where L(θ) is the valve lift amount that may be determined empiricallyby considering cylinder charge motion, combustion stability, minimumvalve opening and closing duration, and emissions. The desired valvelift amount may be stored in tables or functions that may be indexed byengine operating conditions, for example.

The volume of intake mixture at IVC may be determined by the followingequation:

$V_{i,{I\; V\; C}} = \frac{V_{a,{I\; V\; C}} - {\left( {1 - F_{e}} \right) \cdot V_{r,{I\; V\; C}}}}{f_{air}}$

Where f_(air) is the proportion of air in the intake mixture, V_(a,IVC)is the cylinder volume occupied by air at IVC as describe above, andF_(e) is the fraction of burned gas in the exhaust manifold that can bedetermined by methods described in literature. For stoichiometric orrich conditions F_(e) can be set equal to one. F_(air) can be determinedfrom:

$f_{air} = \frac{1}{1 + \frac{1}{A\; F\; R} + F_{i}}$

Where AFR is the air fuel ratio and F_(i) is the fraction of burned gasin the intake manifold. F_(i) can be estimated by methods described inliterature. The volume occupied by the total mixture at IVC can bedetermined by the equation:

V _(IVC) =V _(i,IVC) −V _(el) +V _(r,IVC)

Where V_(cl) is the cylinder clearance volume, V_(r,IVC) is the residualvolume at IVC, and V_(IVC) is the total cylinder volume at IVC. Thevolume occupied by residual gas at IVC can be described by:

$V_{r,{I\; V\; C}} = {\frac{T_{I\; V\; C}}{T_{exh}} \cdot \frac{P_{exh}}{P_{I\; V\; C}} \cdot \left( {V_{r,{EVC}} + V_{cl}} \right)}$

Where T_(IVC) is the temperature at IVC that may be approximated by aregression of the form T_(IVC)=f(N,m_(f),θ_(ov)). Where N is enginespeed, m_(f) is fuel flow rate, and θ_(ov) valve overlap. T_(exh) istemperature in the exhaust manifold, P_(exh) is pressure in the exhaustmanifold, V_(cl) is cylinder clearance volume, P_(IVC) is pressure inthe cylinder at IVC, and V_(r,EVC) is the residual volume at EVC. In oneexample, where IVO is before EVC and where EVC and IVO are after TDC,V_(r,EVC) can be described by:

$V_{r,{EVC}} = {\int{\frac{A_{e}(\Theta)}{{A_{i}(\Theta)} + {A_{e}(\Theta)}}{{V(\Theta)}}}}$

Where the integral is evaluated from IVO to EVC, and where A_(i) andA_(e) are the effective areas of the intake and exhaust valves forθε(θ_(IVO), θ_(EVC)) that may be determined in the same manner asdescribed above for A_(valve)(Θ) . In this example, a predeterminedvalve lift can be used to describe an effective area of the intake valveopening. The intake valve area may be varied as a function of Θ so thatfor a certain cylinder temperature and pressure, a desired mass fractionof EGR may be trapped in a cylinder displacing a volume V_(r,EVC).

The cylinder volume minus the clearance volume at IVC can then be usedto determine intake valve closing position by solving the followingequation for θ:

$V_{\Theta} = {\frac{\pi \; B^{2}}{4}\left\lbrack {r + C - \left( {{{C \cdot \cos}\; \Theta} + \sqrt{r^{2} - {{C^{2} \cdot \sin^{2}}\Theta}}} \right)} \right\rbrack}$

In this way, valve lift, IVC, and IVO can be determined by accountingfor EGR and desired air amount.

In addition, engine fuel can also be adjusted in step 602 so that adesired exhaust air-fuel mixture may be achieved. During some conditionsthe exhaust gas air-fuel mixture may be lean while during otherconditions the mixture may be rich or stoichiometric. For example, if anengine is stopped after being warm and if there may be a higherprobability that the engine will restart, as with some hybrid vehicleapplications, the air-fuel mixture can be commanded to stoichiometry sothat the probability of disturbing an exhaust system catalyst may bereduced. The routine proceeds to step 603.

In step 603, a decision is made to continue reducing cylinder air amountor to proceed to a step that can stop fuel flow to the engine. If thevalve timing determined from step 602 inducts a cylinder air amount thatmay not be sufficient for a desired level of combustion stability theroutine proceeds to step 604. If the cylinder air amount may be above anamount that supports a desired level of combustion stability, theroutine returns to step 602.

In step 604, fuel flow to the engine or cylinder can be stopped. Becausecylinder air amount may be adjusted to a level that may be below adesired combustion stability limit, it can be desirable to stop fuelflow to the engine or to individual cylinders. Fuel flow may be stoppedwhen at least one cylinder air amount may be below a desired amount orfuel may be stopped in individual cylinders as the respective cylinderair amount may be reduced below a desired amount. If fuel flow isstopped on an individual cylinder basis, the valve lift/phase maycontinue to be adjusted in cylinders that may not be below a desiredcylinder air amount.

Spark may also be deactivated in step 604, preferably after the latestair-fuel mixture is combusted. Spark may be deactivated immediatelyafter combusting the latest injected fuel or it may be deactivated aftera predetermined number of cylinder cycles. By delaying sparkdeactivation, it may be possible to combust fuel that may be drawn intothe cylinder from an intake manifold puddle, for example. The routinecontinues to step 605.

In step 605, valve lift and/or phase can be evaluated to determine iffurther adjustments may be desired. If the valve lift and/or phase arenot at a desired low flow position the routine returns to step 604 wherefurther valve actuator adjustment may be commanded. If the valve liftand/or phase are at a desired low flow position, the routine can proceedto step 606.

In step 606, valve lift and/or phase can be held in a low lift and/orphase position. Typically, variable event valve actuators can bedesigned with a minimum lift and/or phase position. In this position,the valve lift may be zero or some fraction of the total available liftamount. Valve phase may be advanced or retarded relative to TDC, forexample. And alternatively, electrically actuated valves may be held ina position (e.g., closed) or at a desired phase by a valve controllercommands. Consequently, in this step, valve operating commands can bestructured based on the actuator design so that a reduced flow,including zero flow, may pass through the cylinder as the enginedecelerates to zero speed.

By commanding the valves to a reduced lift and/or to a phase thatreduces cylinder flow, oxygen pumped through the engine to a catalystmay be reduced. As mentioned above, reducing oxygen flow to a catalystcan improve engine emissions during a subsequent start since thecatalyst state may maintain a desirable level of oxidants. By regulatingthe amount of oxygen that may be stored in a catalyst, catalyticreaction sites may be available for both oxidation and reductionreactions, thereby increasing the possibility of converting HC, CO, andNOx during a subsequent restart. On the other hand, if the amount ofoxygen stored on the catalyst is greater than desired, the catalyst NOxreduction capacity may be diminished since some reduction sites may beoccupied by oxygen. The routine proceeds to step 607.

In step 607, engine speed is compared to a predetermined level. Ifengine speed is below a predetermined level, vlv_lim, the routine exits.When the routine exits, the valve actuators may be set to a desiredposition so that air flow and the cooling and the oxygen that it canbring to a catalyst may be reduced. If engine speed is above thepredetermined level, the routine returns to step 606.

Referring to FIG. 7, an example flow chart of an engine startingsequence for an engine having a variable event valvetrain is shown.

After an engine is stopped, oxygen flow to a catalyst can alter thecatalyst chemical or physical state so that engine emissions mayincrease during a subsequent restart. That is, it can be possible tostop an engine when catalyst chemistry may be favorable to convertinghydrocarbons, carbon monoxide, and oxides of nitrogen. However, allowingthe amount of oxygen stored in the catalyst to increase during an enginestop period or during the starting can reduce the catalyst NOxconversion efficiency since oxygen flow to a catalyst can reducecatalyst temperature and since stored oxygen may be preferentially usedto oxidize hydrocarbons and carbon monoxide. Consequently, NOx may passthrough the catalyst without being reduced because potential reductionsites may be occupied by oxygen that may have been pumped through theengine. The method of FIG. 7 may reduce engine emissions by reducing theamount of oxygen pumped through an engine during a start.

Continuing with FIG. 7, in step 701, the routine determines if a requestto start the engine has been made. If there has been no request to startthe engine the routine can exit. The routine of FIG. 7 can also berepeatedly executed at predetermined times or in response to an engineor controller operating event so that valve adjustments may be readilymade. If there is a request to start the engine the routine proceeds tostep 703.

During this step the valves may also be commanded to an initialposition, a low lift and/or a predetermined valve phase where flowthrough the cylinder may be reduced when the engine rotates, if desired.However, valves may be held in a low flow position (e.g., closing allvalves, closing intake valves, or closing exhaust valves) while theengine is stopped to further reduce oxygen flow to a catalyst.

In step 703, the routine increases engine speed and determines when tobegin adjusting valve lift and/or phase. In one example, the electricmotor of a hybrid vehicle uses at least a portion of the electric motorpower to rotate an internal combustion engine. The engine speed can beramped up to a desired speed in a linear manner, if desired.

The valve adjustment timing schedule can be resolved by subtracting thetime for the valve actuator to move from an initial position to adesired position, vev_ΔT, from the time to accelerate the engine fromstop to a desired speed, ΔT. That is, the valve adjustment starting timecan be expressed by the following equation:

T _(—) strt _(—) vlv=ΔT−vev _(—) ΔT

FIG. 4 can be used to illustrate this method of valve actuator control.The starting sequence begins at the time represented by vertical line T₄and the engine reaches a desired speed at time T₇. This is the time ΔT.The time to move the valve actuator to a desired position is the timebetween T₅ and T₇, vev_ΔT, and may be a function of engine oiltemperature and/or battery voltage, for example. The engine rotates fromT₄ to T₅ before the valve actuator begins to move to the desiredposition. In this way, the air flow through the engine during an enginestart may be reduced since the valves may be commanded to a low flowposition while the engine speed is increasing and cylinders may bepumping air through the engine. The routine continues to step 705.

It is also possible to adjust exhaust valve lift/phase during a start sothat air pumped through an engine may be reduced. For example, exhaustvalve lift may be initially set to a zero or low lift position and thenincreased as engine speed increases. By reducing exhaust valve lift atlow or zero engine speed, less air may be pumped into the exhaustmanifold for at least a portion of the starting interval. As enginespeed increases, and as engine position is determined, exhaust valvelift may be increased so that combusted gases may be expelled into theexhaust system. This method may be more beneficial after a longer engineoff period than after a shorter engine off period since fewer exhaustresiduals may be trapped within the cylinder.

In step 705, engine speed continues to increase and the variable eventvalvetrain may be held at a constant actuator position. That is, thevalve may be held at a minimum or flow reducing position. This methodcan allow the engine to reach a desired speed with reduced cylinderflow. The routine proceeds to step 707.

In step 707, a decision can be made to begin adjusting the variableevent valve actuator. If the valve starting time has been exceeded theroutine proceeds to step 709. If not, the routine returns to step 705.

In step 709, the variable event valvetrain may be adjusted while enginespeed is being increased. Cylinder air flow may be increased byadjusting the valve lift amount and/or the valve phase. The adjustmentmay be at a constant or variable rate depending on objectives.Furthermore, the adjustment rate may be based on time (e.g.,milli-meters per second) or engine speed. Alternatively, the valve liftand/or phase may be adjusted to produce a desired cylinder or enginetorque or to induct a desired cylinder air charge by the methoddescribed in FIG. 6. The routine proceeds to step 710.

In step 710, a decision can be made to continue valvetrain adjustment orto proceed to step 711 based on engine speed. If the engine speed isbelow a predetermined desired amount the routine returns to step 709. Ifthe engine speed is above a predetermined amount the routine continuesto step 711.

In step 711, a decision can be made to continue valvetrain adjustment orto proceed to step 713. If the variable event valvetrain is at a desiredposition the routine proceeds to step 713. If not, the routine returnsto step 709.

Note: steps 710 and 711 may be combined into a single step that allowsthe routine to proceed to step 711 if both the engine speed is at adesired level and if the variable event valvetrain is at a desired liftamount or phase. If not, the routine would return to step 709.

In step 713, cylinder fuel can be enabled and the variable eventvalvetrain can be held in position. By delaying fuel until a desiredamount of cylinder air flow may be present, misfires may be reduced.Further, delaying valve adjustment until the engine is at a desiredspeed can reduce the air pumped to a catalyst and may improve engineemissions during the restart.

Cylinder spark can also be reactivated in step 713 so that the injectedfuel can be combusted. The routine proceeds to exit.

In an alternate example, a valve adjustment timing schedule illustratedby FIG. 5 may be used. In this example, the engine can be rotated by astarter motor that may be capable of rotating the engine at lowerspeeds, 300 RPM or less for example.

The starting sequence begins at T₄ and the engine is at a desired speedat T₇. The time to adjust the valve actuator is shown between time T₅and time T₇. In this example, the valve actuator does not begin toadjust the valve lift and/or phase until location T₅. The delay timebetween T₄ and T₅ may be related to the time that it can take tosynchronize the engine controller to the engine position and/or thedelay time may be a function of engine oil temperature and/or batteryvoltage, engine friction, engine speed, and/or another engine relatedvariable. As mentioned above, air flow through the engine during anengine start may be reduced since the valve may be commanded to areduced flow position.

In yet another embodiment, the valve adjustment may begin coincident ordelayed from initial engine rotation by a predetermined amount of time.When the valve actuator reaches a position that can support a desiredlevel of combustion stability and/or a cylinder inducts a desired airamount, the fuel may be enabled.

The method of FIGS. 7 may also be extended to include throttle control.In particular, an electronic throttle may be held closed or at a fixedposition at a start until engine position is determined and/or until apredetermined valve lift amount may be achieved.

Referring to FIG. 8, a flow chart of an example method to deactivate acylinder is shown. This method may be used when an engine transitionsfrom a first number of operating cylinders to a second number ofoperating cylinders, said second number of cylinders less than saidfirst number of cylinders.

In step 801, the routine determines if a request to deactivate acylinder has been made. The deactivation request may be based on engineoperating conditions or the request may come from a hybrid vehiclecontroller, for example. If a request has been made the routine proceedsto step 803. If not, the routine can exit. The routine of FIG. 8 canalso be repeatedly executed at predetermined times or in response to anengine or controller operating event so that valve adjustments may bereadily made.

In step 803, the variable event valvetrain is adjusted so that air flowthrough the cylinder may be reduced. The adjustment may reduce valvelift or alter the valve timing with respect to a crankshaft position.Furthermore, the adjustment rate may be a constant, varied based onengine operating conditions, or based on a predetermined torque orcylinder air charge rate of reduction. The routine proceeds to step 805.

In step 805, the routine decides whether to continue adjusting thevalvetrain or to proceed to the next step. If the valve lift and/orphase has limited the cylinder air flow below the amount necessary for adesired level of combustion stability the routine proceeds to step 807.If cylinder air flow is above the amount necessary for a desired levelof combustion stability the routine returns to step 803.

In step 807, fuel flow is stopped and the valvetrain may continue to beadjusted. By stopping fuel flow to a cylinder when cylinder air flow maybe below an amount that may be necessary to maintain a desired level ofcombustion stability, misfires may be reduced. Furthermore, bycontinuing to reduce the valve lift and/or change the phase, air flowthrough the cylinder may be reduced and may lower the amount of oxygenthat may be pumped to a catalyst during cylinder deactivation.

Spark may also be deactivated in step 807, preferably after the latestair-fuel mixture is combusted. Spark may be deactivated immediatelyafter combusting the latest injected fuel or it may be deactivated aftera predetermined number of cylinder cycles. The routine proceeds to step809.

In step 809, a decision is made to continue adjusting the valve actuatoror to move to step 811. After fuel flow is stopped to a cylinder thevalve actuator may need additional time before reaching a low flowposition and/or additional commands may be issued by engine controller12 so that the valve actuator reaches a low flow position. If the valveactuator has not reached a low flow position the routine returns to step807. If the valve actuator has reached the low flow position the routinecontinues to step 811.

Note: The above-mentioned low flow valve actuator position does notnecessarily have to be a minimum flow position. The low flow positionmay be some fractional amount of the total available flow range.Furthermore, the low flow position may vary with engine operatingconditions, for example.

In step 811, the valve actuator can be held in the low flow position.The valve actuator may be commanded to a fixed position after fuel flowand air flow have been reduced to the cylinder. Alternatively, the valveactuator may be held in position by a mechanical or electromechanicaldevice. By holding the valve actuator at a low flow position catalysttemperature and conversion efficiency may be maintained since fresh airflow through the engine may be reduced. The valve actuator may remain inthe low flow position until the cylinder reactivation sequence begins oruntil a change in cylinder air amount may be desired. The routineproceeds to exit.

The method of FIGS. 6 and 8 may also be extended to include throttlecontrol. Specifically, it may be desirable to control an electronicthrottle such that the throttle can be closed after a request to stop anengine. By closing the throttle it may be possible to reduce the enginestop time since the air charge for desired combustion stability may beachieved sooner.

In the methods described by FIGS. 6-8 it may be desirable to adjustthrottle 125 to further reduce cylinder air flow. The throttle openingposition may be gradually reduced or closed in a single step or in anumber of steps after a request to stop the engine or reduce cylinderair flow has been issued. Also, the throttle and valve adjustment ratesmay be modified in response to barometric pressure and/or humidity, forexample.

As will be appreciated by one of ordinary skill in the art, the routinesdescribed in FIGS. 6-8 may represent one or more of any number ofprocessing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various steps orfunctions illustrated may be performed in the sequence illustrated, inparallel, or in some cases omitted. Likewise, the order of processing isnot necessarily required to achieve the objects, features, andadvantages described herein, but is provided for ease of illustrationand description. Although not explicitly illustrated, one of ordinaryskill in the art will recognize that one or more of the illustratedsteps or functions may be repeatedly performed depending on theparticular strategy being used.

This concludes the description. The reading of it by those skilled inthe art would bring to mind many alterations and modifications withoutdeparting from the spirit and the scope of the description. For example,I3, I4, I5, V6, V8, V10, and V12 engines operating in natural gas,gasoline, diesel, or alternative fuel configurations could use thepresent description to advantage.

1-17. (canceled)
 18. A method to start an internal combustion enginehaving a variable event valvetrain, the method comprising: increasingthe speed of said internal combustion engine during a start from astopped position after a request to start said internal combustionengine; increasing an intake valve lift amount, of at least a cylinderduring said start while said internal combustion engine speed isincreasing; and only enabling fuel flow to said cylinder when saidintake valve lift amount reaches a predetermined level.
 19. The methodof claim 18 wherein said internal combustion engine is at least onepower source in a hybrid vehicle.
 20. The method of claim 18 wherein astarter motor increases said speed of said internal combustion engine.21. (canceled)
 22. The method of claim 18 further comprising deliveringfuel to said cylinder when said engine speed reaches a predeterminedlevel.
 23. (canceled)
 24. The method of claim 18 further comprisingdelivering spark to said engine in response to an engine operatingcondition.
 25. The method of claim 18 wherein the valve lift amount ofsaid at least a cylinder begins from a minimum lift position.
 26. Amethod to start an internal combustion engine having a variable eventvalvetrain, the method comprising: increasing the speed of said internalcombustion engine from a stopped position after a request to start saidinternal combustion engine; adjusting a valve actuator phase from afirst amount to a desired amount of at least a cylinder at least whilesaid internal combustion engine speed is increasing during at least aportion of a engine start; and enabling fuel flow to said cylinder whena desired cylinder air flow is achieved.
 27. A method to start aninternal combustion engine having a variable event valvetrain, themethod comprising: increasing the speed of said internal combustionengine from a stopped position after a request to start said internalcombustion engine to a predetermined speed, while fuel flow is stoppedto said engine and while an intake valve lift amount of at least acylinder is at a first intake valve lift amount; increasing said intakevalve lift amount from said first amount to a second amount, during orafter the time said engine speed reaches said predetermined speed; andenabling fuel flow when a desired cylinder air flow is achieved. 28.(canceled)